Can the PM ignore the Commons on a customs union?

I've been asking MPs of a certain age and endowed with parliamentary memory whether they can think of any precedent for what will happen on Tuesday - namely a government asking MPs to reject serious proposals that would shape the economy and governance of the UK for generations to come, without actually being able to offer those MPs a detailed alternative plan.

Perhaps it may surprise you that everyone I've approached has said that Theresa May is attempting something in the House of Commons without proper precedent - and most of them argue, even a few ardent Brexiters, that what she is doing is wrong.

It is seen as degrading the status of MPs to ask them - as she is - to vote against their consciences, which tell many of them that the UK should stay in the customs union, that it is paramount the Ireland border should remain open and that MPs should be able to force the PM back to the negotiating table if they don't like final Brexit terms, without explaining what her alternative and cunning plan may be.

That is why the resolve of rebel Tory MPs to defy her whip and will is being reinforced.

And there is a serious risk for her that the usual dozen of ultra-Remainy Conservative troublemakers will be joined by colleagues normally seen as May loyalists - but whose patience is being sorely tested by the conspicuous absence of a Brexit trade policy worth the name.

That patience was challenged again by the PM, answering Corbyn at PMQs, when she refused even to give guidance on when a Brexit white paper - designed to set out the government's negotiating position - will be published.

But if Tuesday's votes are in and of themselves taking us into uncharted territory, the aftermath will almost certainly do that too.

Because if May loses the votes she will do her utmost to ignore them.

And in the case of the customs vote, that would be theoretically possible for her, since its status is largely symbolic.

But if she is defeated, there could no longer be any doubt that the will of Parliament is for the UK to stay in a customs union.

So with confidence in politicians and our polity so fragile, the PM would presumably think twice before treating Parliament as a tinpot talking shop whose views are contemptible.

But how she avoids doing that, without simultaneously conceding that the kernel of her Brexit strategy is bankrupt, would take some doing.

Brexit was supposed to be about taking back control.

Right now, either Parliament or the PM and her executive are in danger of being castrated.